It is well known that the strength and stiffness of a material may be improved by addition of strong and/or stiff reinforcements such as particles, chopped fibers or long fibers to form a composite structure. This practice is applicable to many matrix materials including metals, ceramics and polymers, but is frequently employed with polymer matrices, which offer lesser strength and stiffness, but lower density, than either ceramics or metals. Thus such a reinforced polymer structure may offer good strength and stiffness in combination with low mass.
If low mass is of particular concern, the composite mass may be further minimized by using a higher strength and low density fiber reinforcement in addition to a low density polymer matrix. Minimizing the mass of the composite will increasingly depend on minimizing the mass of the reinforcing higher strength fibers as performance demands accelerate the trend toward increasing fiber volume fractions. One, commonly used, high strength, low density reinforcement is carbon fibers.
In some applications, consideration of only a limited number of engineering attributes, like stiffness, strength or mass of a reinforced polymer may be sufficient to determine whether or not it is suited for a particular purpose. In many more applications however, a much broader range of engineering properties, characteristics and behaviors must be taken into account. For example, in assemblies where multiple materials are employed, compatibility, in performance or appearance, with other, different materials may be an issue. A case in point occurs in some automotive applications, where an individual reinforced composite component or structure, for example a hood, may be located adjacent to a sheet metal component, for example a fender. It is intended that the painted appearance of all the components should match.
Reinforcing fibers may be used as discrete reinforcements but in many cases, the carbon fibers are arranged in organized groupings. For example, in some carbon fiber reinforced polymers the generally cylindrical carbon fibers are first organized into tows. These are assemblages of continuous or near-continuous, untwisted fibers loosely gathered together. Tows often adopt a ribbon-like configuration and may be generally elliptical in cross-section, and are, optionally, lightly secured using an epoxy sizing. Such tows may then be woven into any desired 2-dimensional pattern to form a reinforcing fabric or sheet. While individual fibers may have a diameter of at least a micrometer or so, a more typical dimension is between 10 and 20 micrometers. A tow, by contrast, which may contain from about 1 to 50 thousand fibers may range in width from between 1.0 to 10 millimeters, or, equivalently, of between 1000 and 10,000 micrometers. Typical weave patterns may be somewhat coarse with adjacent parallel tows being spaced between 1 and 5 millimeters apart.
Fabrication of a component begins with impregnating the woven sheets with a polymer resin, which, for ease of handling, is often partially cured or B-staged but remains flexible and conformable. Such a resin-impregnated sheet is termed a prepreg.
In volume production, heated multipart molds are frequently employed. Such molds are capable of receiving the prepreg and shaping it, under pressure, to the desired article shape before the mold is heated. The mold temperature may then be increased to raise the prepreg temperature and cure the resin and form a composite article. The mold parts are separable for loading the prepreg and for extracting the cured article and the various mold parts, when assembled into their operating configuration, define a die cavity corresponding to the desired article geometry.
Such mold may accept a single prepreg but, more frequently, multiple resin-impregnated sheets are placed one atop the other to form a layup. In assembling the layup, the sheets, and their associated reinforcements, may be displaced or rotated with respect to one another to reduce in-plane property directionality in the finished component. The layup may be fabricated in the mold or may be prepared off-line and placed in the mold only when fully assembled.
The coefficients of thermal expansion (CTE) of the polymer and reinforcement are significantly different. In addition, most carbon fibers have an anisotropic CTE, exhibiting one CTE along their axis and a second CTE across their diameter, assuring that there will be at least some mismatch in CTE between the fiber and the larger and more isotropic CTE of the polymer. During cool-down of the composite article this mismatch in CTE will induce stresses which will produce print through of the weave pattern, that is, surface distortions which mimic the weave pattern spacing. The regions between the fiber tows will be depressed compared to the more elevated regions overlying the fiber tows.
These surface distortions, or print through of the fiber pattern, are clearly visible, particularly on a painted surface, and are unacceptable on any ‘show surface’ which may be viewed by a customer. Thus many viewable fiber composite components require extensive finishing, typically priming and sanding, prior to final painting.
There is therefore a need for a method of suppressing or minimizing print through in fiber reinforced composite panels to minimize the need for remedial surface treatments prior to assembly and painting of the panels.